'In this campaign we are the reasonable people. I think in the 'Leave' side, they are the extremists on this'

Alan Johnson has branded pro-Brexit MPs "extremists" in an attack that was angrily rejected by former Tory cabinet minister Iain Duncan Smith who accused the former Labour home secretary of demeaning the referendum debate.

Johnson, who is leading Labour's 'In' campaign, also said he would be happy to take on Boris Johnson in a head-to-head live TV debate if David Cameron dodged it.

Following the launch of Labour's pro-EU battle bus on Tuesday morning, Johnson said the the Brexit campaigners displayed "a certain mentality that is not rational and not balanced".

"It’s an extreme view that there is absolutely nothing good about the EU at all," he said. "They can’t find anything good to say, I think that’s extreme."

"I think the majority of the British people have a rational balanced moderate approach to this question."

Johnson said those in the 'Remain' campaign had a more "nuanced balanced approach" to the EU. Jeremy Corbyn, he said, "doesn’t think it's perfect neither does David Cameron".

"We can all find things that are wrong with the EU. But they can’t find anything that’s right," he said.

"In this campaign we are the reasonable people. I think in the 'Leave' side, they are the extremists on this."

"I think it’s extreme, not to take the view we ought to leave, but the view you can not find anything good to say about an institution that has done many good things."

Duncan Smith, a leading Brexit campaigner, hit back. "I have to say those people in Remain really need to stop throwing threats and ridiculous terms like that around because it just demeans them and it demeans the debate," he told The Huffington Post.

The former work and pensions secretary, who delivered his own pro-Brexit speech today, said: "I don’t know in what world it’s extreme to want to have your democracy back, the power over what you do, control over your laws and the right to make decisions about your people, elected by British people, rejected by British people when you get it wrong.

"If someone wants to tell me that’s an extreme position then I want to know what defines that.

"It’s not extreme to want democratic government in your country, to be responsible to the electorate and to make the laws that best help them rather than have 60 per cent of those laws made in Brussels."

PA

Johnson, who will tour the UK in a Labour pro-EU bus to make the case for 'Remain', said it was not "overblown" for Cameron to argue Brexit could lead to war.

And he said Cameron's voice was extremely important in the debate. "A British prime minister, the elected prime minister in a democracy, actually carries weight with people who don’t support his political party. His position does mean something. And it meant something when Harold Wilson was recommending staying 'In' in 1975. It means something now."

LIVE TV REFERENDUM DEBATE

Alan Johnson has put himself forward to take on Boris Johnson or Michael Gove in a live TV EU referendum debate.

On Sunday, George Osborne suggested David Cameron would not take go head-to-head with Boris or Gove as it would turn the campaign into a "Tory soap opera".

Asked today if he would step in to argue the case for the EU, Johnson said: "I would yeah, sure."

He added that Gordon Brown, who is due to make an intervention in the campaign this week, would also be up to the task. "Gordon would be a good voice," he said.

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: Labour leader Jeremy Corbyn (C) walks past party activists as he launches the Labour In for Britain EU campaign battle bus on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

Kirsty Wigglesworth/AP

Jeremy Corbyn, Leader of Britain's Labour Party poses for a photograph with young supporters at the launch of 'Labour In for Britain', ahead of June's EU referendum, in London, Tuesday, May 10, 2016. (AP Photo/Kirsty Wigglesworth)

LEON NEAL via Getty Images

Alan Johnson (2nd L), the Chair of Labour In for Britain speaks beside deputy leader of the Labour Party, Tom Watson (3rd L), shadow Minister for Young People and Voter Registration, Gloria De Piero (3rd R) and Britain's opposition Labour Party leader Jeremy Corbyn (2nd R), in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

Anadolu Agency via Getty Images

LONDON, UNITED KINGDOM - MAY 10: Former home secretary Alan Johnson delivers a speech in front of the 'Labour In For Britain' campaign bus during a launch in central London, United Kingdom on May 10, 2016. (Photo by Ray Tang/Anadolu Agency/Getty Images)

Anadolu Agency via Getty Images

LONDON, UNITED KINGDOM - MAY 10: Former home secretary Alan Johnson (2nd L), Deputy leader of the Labour Party, Tom Watson (3rd L), shadow Minister for Young People and Voter Registration, Gloria De Piero (4th L) and British Labour party leader Jeremy Corbyn (5th L) attend a launch in front of the 'Labour In For Britain' campaign bus in central London, United Kingdom on May 10, 2016. (Photo by Ray Tang/Anadolu Agency/Getty Images)

LEON NEAL via Getty Images

Shadow Minister for Young People and Voter Registration, Gloria De Piero (4th L), speaks beside deputy leader of the Labour Party, Tom Watson (3rd L), Alan Johnson (2nd L), the Chair of Labour In for Britain, and Britain's opposition Labour Party leader Jeremy Corbyn, in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

Anadolu Agency via Getty Images

LONDON, UNITED KINGDOM - MAY 10: Former home secretary Alan Johnson (2nd L), Deputy leader of the Labour Party, Tom Watson (3rd L), shadow Minister for Young People and Voter Registration, Gloria De Piero (4th L) and British Labour party leader Jeremy Corbyn (5th L) attend a launch in front of the 'Labour In For Britain' campaign bus in central London, United Kingdom on May 10, 2016. (Photo by Ray Tang/Anadolu Agency/Getty Images)

Anadolu Agency via Getty Images

LONDON, UNITED KINGDOM - MAY 10: British Labour party leader Jeremy Corbyn (C) poses for picture with party activists in front of the 'Labour In For Britain' campaign bus at a launch in central London, United Kingdom on May 10, 2016. (Photo by Ray Tang/Anadolu Agency/Getty Images)

LEON NEAL via Getty Images

Britain's opposition Labour Party leader Jeremy Corbyn gestures in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

LEON NEAL via Getty Images

Labour Party supporters pose for a photograph in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

LEON NEAL via Getty Images

Britain's opposition Labour Party leader Jeremy Corbyn and shadow Minister for Young People, Gloria De Piero (L) stand in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

LEON NEAL via Getty Images

Britain's opposition Labour Party leader Jeremy Corbyn and shadow Minister for Young People, Gloria De Piero (L) stand in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: Labour leader, Jeremy Corbyn (C), speaks as he launches the Labour In for Britain EU campaign battle bus with Gloria De Piero (2L), Shadow Minister for Young People and Voter Registration and Deputy Leader of the Labour Party, Tom Watson (L), on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

LEON NEAL via Getty Images

Britain's opposition Labour Party leader Jeremy Corbyn (R) speaks beside shadow Minister for Young People and Voter Registration, Gloria De Piero (2nd R), deputy leader of the Labour Party, Tom Watson (2nd L) and Alan Johnson (L), the Chair of Labour In for Britain, in front of the 'Labour In For Britain' campaign bus at a launch in central London, on May 10, 2016.
The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.
/ AFP / LEON NEAL (Photo credit should read LEON NEAL/AFP/Getty Images)

John Stillwell/PA Wire

(From second left) Former home secretary Alan Johnson, Deputy Leader of the Labour Party, Tom Watson, Shadow Minister for Young People, Gloria De Piero, and Labour Party leader Jeremy Corbyn join activists as they launch the Labour In for Britain EU campaign battlebus on the South Bank on London.

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: Labour leader,Jeremy Corbyn (2R) speaks as he launches the Labour In for Britain EU campaign battle bus with Gloria De Piero (2L), Shadow Minister for Young People and Voter Registration and Deputy Leader of the Labour Party, Tom Watson (L) on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: Labour leader Jeremy Corbyn (C) speaks as he launches the Labour In for Britain EU campaign battle bus with Gloria De Piero, Shadow Minister for Young People and Voter Registration, Deputy Leader of the Labour Party, Tom Watson and Alan Johnson, the Chair of Labour In for Britain, on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: Labour leader Jeremy Corbyn is seen with party activists as he launches the Labour In for Britain EU campaign battle bus on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

Carl Court via Getty Images

LONDON, ENGLAND - MAY 10: An activist shelters under an umbrella in front of the Labour In for Britain EU campaign battle bus ahead of its launch by Labour leader Jeremy Corbyn on May 10, 2016 in London, England. Mr Corbyn made a plea for workers' rights and vowed to stop the TTIP trade deal as he set off on a six-week trip around Britain. (Photo by Carl Court/Getty Images)

John Stillwell/PA Wire

Labour party leader Jeremy Corbyn launches the Labour In for Britain EU campaign battlebus on the South Bank on London.

Kirsty Wigglesworth/AP

Jeremy Corbyn, Leader of Britain's Labour Party wipes his eye at the launch of 'Labour In for Britain', in front of the EU campaign bus, ahead of June's EU referendum, in London, Tuesday, May 10, 2016. (AP Photo/Kirsty Wigglesworth)

Kirsty Wigglesworth/AP

Jeremy Corbyn, Leader of Britain's Labour Party speaks at the launch of 'Labour In for Britain', in front of the EU campaign bus, ahead of June's EU referendum, in London, Tuesday, May 10, 2016. (AP Photo/Kirsty Wigglesworth)